Un-redirect script output

Question

I have a script with an error handler and output redirection to syslog:

#!/bin/bash

error_handler() {
  echo "Unexpected error ${1} in ${2}, line ${3}, ${4}(): ${5}"
  exit "${1:-1}"
}
set -eETuo pipefail
trap 'error_handler "$?" "$0" "$LINENO" "${FUNCNAME[0]:-main}" "$BASH_COMMAND"' ERR

exec > >(logger -p local0.err -t foo -e --id="$$") 2> >(logger -p local0.err -t foo -e --id="$$")

# rest of the script here

This works fine and logs both regular output as well as errors to syslog. However, now I would like to run the script as a cron job and get notified when the job fails. Cron by default sends notifications whenever a cron job produces output, but the redirection causes the script to be completely silent.

I could add ... | tee /dev/tty or ... >/dev/tty to error_handler() so that termination messages would appear on the console, but cron jobs don't have a terminal, so this would only work when running the script interactively.

Of course I could change the script so that the error handler itself would send the e-mail

from="root@$(hostname -f)"
to='me@example.org'
echo "Unexpected error ${1} in ${2}, line ${3}, ${4}(): ${5}" |
  tee /dev/stderr |
  mailx -r "$from" -c '' -s "$0 failed" "$to"

but that would require an additional program for something that cron can already do by itself.

How can I get my script to generate error output for cron (to trigger failure e-mails) while still logging the full output to syslog?

Answer 1

You need to preserve the un-redirected stdout in a different file descriptor before redirecting it to syslog:

exec 3>&1 > >(logger ...) 2> >(logger ...)

Note: the order in which the redirections occur in this line matters!

With the un-redirected stdout preserved as fd 3 you can then restore it in your error handler:

error_handler() {
  exec 1>&3
  echo "Unexpected error ${1} in ${2}, line ${3}, ${4}(): ${5}" | tee /dev/stderr
  exit "${1:-1}"
}

The tee command duplicates the termination message to stderr (which is still redirected to syslog), so that the message goes to both cron and syslog.